Why Does Rijndael Decryption Fail with 'Padding is Invalid and Cannot Be Removed'?

Rijndael Decryption Error: Invalid Padding

Decrypting data encrypted with the Rijndael algorithm can sometimes result in a "Padding is invalid and cannot be removed" error. This error arises from an inconsistency between the padding used during encryption and the padding expected during decryption.

Rijndael operates on 128-bit blocks. Padding ensures that the last block is always a full 128 bits. If this padding is incorrect during decryption, the process fails.

Understanding Padding's Role

Padding is crucial for data block alignment and security. It prevents certain cryptographic attacks. PKCS#7 padding is commonly used with Rijndael, adding extra bytes to the end of the data to fill the last block.

The Solution: Consistent Padding

The key to resolving this error is to explicitly define the padding mode in both the encryption and decryption processes. This ensures consistency.

For encryption, explicitly set PKCS#7 padding:

<code class="language-csharp">key.Padding = PaddingMode.PKCS7;</code>

Similarly, for decryption, specify PKCS#7 padding:

<code class="language-csharp">exml.Padding = PaddingMode.PKCS7;</code>

By explicitly defining the padding mode, you eliminate the padding mismatch and enable successful decryption.

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